Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Multi-Carrier or Carrier Aggregation Concept
To enhance peak-rates within a technology, multi-carrier or carrier aggregation solutions are known. A radio network node and a UE transmit and/or receive signals over a carrier, also known as carrier frequency. For example, it is possible to use multiple 5 MHz carriers in High Speed Packet Access (HSPA) to enhance the peak-rate within a HSPA network. Similarly in LTE for example multiple 20 MHz carriers or even smaller carriers (e.g. 5 MHz) may be aggregated in the UL and/or on DL. Each carrier in multi-carrier or carrier aggregation system is generally termed as a Component Carrier (CC) or sometimes is also referred to a cell. In simple words the CC means an individual carrier in a multi-carrier system. The term Carrier Aggregation (CA) is also called, e.g. interchangeably called, multi-carrier system, multi-cell operation, multi-carrier operation, multi-carrier transmission and/or reception. This means the CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is the Primary Component Carrier (PCC) or simply primary carrier or even anchor carrier. The remaining ones are called Secondary Component Carrier (SCC) or simply secondary carriers or even supplementary carriers. Generally the primary or anchor CC carries the essential UE specific signaling. The primary CC exists in both uplink and direction CA. The network may assign different primary carriers to different UEs operating in the same sector or cell.
Therefore the UE has more than one serving cell in downlink and/or in the uplink: one primary serving cell and one or more secondary serving cells operating on the PCC and SCC respectively. The serving cell is interchangeably called as Primary Cell (PCell) or Primary Serving Cell (PSC). Similarly the secondary serving cell is interchangeably called as Secondary Cell (SCell) or secondary serving cell (SSC). Regardless of the terminology, the PCell and SCell(s) enable the UE to receive and/or transmit data. More specifically the PCell and SCell exist in DL and UL for the reception and transmission of data by the UE. The remaining non-serving cells on the PCC and SCC are called neighbor cells.
The CCs belonging to the CA may belong to the same frequency band, also known as intra-band CA, or to different frequency band, inter-band CA, or any combination thereof. E.g. 2 CCs in band A and 1 CC in band B. The inter-band CA comprising carriers distributed over two bands is also called as Dual-Band-Dual-Carrier-(DB-DC-) High Speed Downlink Packet Access (HSDPA) in HSPA or inter-band CA in LTE. Furthermore the CCs in intra-band CA may be adjacent or non-adjacent in frequency domain, also known as intra-band non-adjacent CA. A hybrid CA comprising of intra-band adjacent, intra-band non-adjacent and inter-band is also possible. Using carrier aggregation between carriers of different technologies is also referred to as “multi-Radio Access Technology (RAT) carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, the carriers from WCDMA and LTE may be aggregated. Another example is the aggregation of LTE and CDMA2000 carriers. For the sake of clarity the carrier aggregation within the same technology as described can be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
The CCs in CA may or may not be co-located in the same site or base station or radio network node, e.g. relay, mobile relay etc. For instance the CCs may originate, i.e. transmitted and/or received at different locations, e.g. from non-located BS or from BS and RRH or RRU. The well-known examples of combined CA and multi-point communication are DAS, RRH, RRU, CoMP, multi-point transmission/reception etc.
New Carrier Type
In NCT mandatory transmissions are minimized to achieve improved user and system throughput, improved energy efficiency, and improved spectrum access and flexibility. In the following some of the main design features of NCT are listed.
Enhanced Synchronization Signal
In NCT, there are fewer resources used for common reference signals within the cell. So instead of transmission of Common Reference Signals (CRS), which is transmitted on two ports, and every subframe, Extended Synchronization Signal (ESS) which is transmitted only on port 0 and every 5th subframes is used. A port is also known as an antenna port, which is an entity used for transmitting radio signals. The reduction in overhead achieved by using the ESS is shown in FIG. 1. FIG. 1 depicts Legacy Carrier Type (LCT) with CRS, and NCT with ESS signal. In FIG. 1, the dotted lines represent the reference signals transmitted. In NCT the reference signal may also be sent over shorter Bandwidth (BW) i.e. over fewer Resource Blocks (RB)s than the cell BW. Furthermore the synchronization signal configuration in a cell operating using NCT may also be different compared to that in LCT i.e. location of PSS and/or SSS signals may be different.
ePDCCH
In LTE, control information, paging and random access responses are transmitted using the Physical Downlink Control Channel (PDCCH). The PDCCH is transmitted in the first few symbols of a subframe of duration 1 ms and over the entire BW. In each subframe the PDCCH may occupy one to three Orthogonal Frequency Division Multiplexing (OFDM) symbols out of fourteen symbols (Four symbols may be used when the carrier bandwidth is only 1.4 MHz). In LTE TS 36.211, Release 11, an additional control mechanism, the enhanced PDCCH (ePDCCH), has been defined for transmitting control information that is specific to a UE, also referred to as UE-specific information. In NCT ePDCCH is used which is transmitted across symbols and over a limited BW. Information that is common to multiple UEs, also referred to as common information, is still transmitted using the legacy PDCCH.
FIG. 2 illustrates LCT with CRS, and NCT with ESS signal. CRS is marked with downward diagonal lines, and ESS is marked with upward diagonal lines.
UE-Specific Carrier Bandwidth
In LTE TS 36.211 Release-11, each UE is informed of the carrier BW and all UEs must support the complete system BW. This is partly because the PDCCH spans over the entire carrier BW. As stated earlier, one of the design goals in LTE Release 12 for the NCT is to enable UEs that support a system BW that is lower than the NCT BW. In order to do this, a UE should be able to perform all functions including initial system access on a subset of the PRBs being transmitted from the carrier. Since, the new carrier type does not use the PDCCH and only uses the ePDCCH which can be deployed in a subset of the total available PRBs, this is possible on the new carrier.
Measurements
Radio Resource Management (RRM) Measurement
Several radio related measurements are used by the UE or the radio network node to establish and keep the connection, as well as ensuring the quality of a radio link.
The measurements are used in Radio Resource Control (RRC) idle state operations such as cell selection, cell reselection e.g. between Evolved Universal Terrestrial Radio Access Networks (E-UTRANs), between different RATs, and to non-3GPP RATs, and Minimization of Drive Test (MDT), and also in RRC connected state operations such as for cell change e.g. handover between E-UTRANs, handover between different RATs, and handover to non-3GPP RATs.
The UE has to first detect a cell and therefore cell identification e.g. acquisition of a Physical Cell Identity (PCI), is also a signal measurement. The UE may also have to acquire the Cell Global ID (CGI) of a UE.
In HSPA and LTE the serving cell can request the UE to acquire the system information of the target cell. More specifically the SI is read by the UE to acquire the CGI, which uniquely identifies a cell, of the target cell. The UE may also be requested to acquire other information such as Closed Subscriber Group (CSG) indicator, CSG proximity detection from the target cell.
The UE reads the SI of the target cell e.g. intra-, inter-frequency or inter-RAT cell, upon receiving an explicit request from the serving network node via RRC signaling e.g. from a Radio Network Controller (RNC) in HSPA or eNode B in case of LTE. The acquired SI is then reported to the serving cell. The signaling messages are defined in the relevant HSPA and LTE specifications.
In order to acquire the System Information (SI) which contains the CGI of the target cell, the UE has to read at least part of the SI including master information block (MIB) and the relevant System Information Block (SIB) as described later. The terms SI reading/decoding/acquisition, CGI/ECGI reading/decoding/acquisition, CSG SI reading/decoding/acquisition are interchangeably used but have the same or similar meaning. In order to read the SI to obtain the CGI of a cell the UE is allowed to create autonomous gaps during DL and also in UL. The autonomous gaps are created for example at instances when the UE has to read MIB and relevant SIBs of the cell, which depends upon the RAT. The MIB and SIBs are repeated with certain periodicity. Each autonomous gap is typically 3-5 ms in LTE and UE needs several of them to acquire the CGI.
Signal Measurements
The Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) are the two existing measurements used for at least RRM such as for mobility, which include mobility in RRC connected state as well as in RRC idle state. The RSRP and RSRQ are also used for other purposes such as for enhanced cell ID positioning, minimization of drive test etc.
The RSRP measurement provides cell-specific signal strength metric at a UE. This measurement is used mainly to rank different LTE candidate cells according to their signal strength and is used as an input for handover and cell reselection decisions. CRS are used for RSRP measurement. These reference symbols are inserted in the first and third last OFDM symbol of each slot, and with a frequency spacing of 6 subcarriers. Thus within a resource block of 12 subcarriers and 0.5 ms slot, there are 4 reference symbols.
The RSRQ is a quality measure which is the ratio of the RSRP and carrier Received Signal Strength Indicator (RSSI). The latter part includes interference from all sources e.g. co-channel interference, adjacent carriers, out of band emissions, noise etc.
The UE depending upon its capability may also perform inter-RAT measurements for measuring on other systems e.g. HSPA, Global System for Mobile Communications (GSM)/GSM EDGE Radio Access Network (GERAN), Code Division Multiple Access (CDMA) 2000 Single-Carrier Radio Transmission Technology (1×RTT) and High Rate Packet Data (HRPD) etc. Examples of inter-RAT radio measurements which can be performed by the UE are Common Pilot Channel (CPICH) Received Signal Code Power (RSCP) and CPICH Ratio of chip Energy to Noise (Ec/No) i.e. CPICH received signal quality, for inter-RAT UTRAN, GERAN carrier RSSI for inter-RAT GSM and even pilot strength measurements for CDMA2000 1×RTT/HRPD. Wherein EDGE is the abbreviation for Enhanced Data Rates for GSM Evolution
In RRC connected state the UE can perform intra-frequency measurements without measurement gaps. However as a general rule the UE performs inter-frequency and inter-RAT measurements in measurement gaps unless it is capable of performing them without gaps. To enable inter-frequency and inter-RAT measurements for the UE requiring gaps, the network has to configure the measurement gaps. Two periodic measurement gap patterns both with a measurement gap length of 6 ms are defined for LTE:                Measurement gap pattern #0 with repetition period 40 ms        Measurement gap pattern #1 with repetition period 80 ms        
The measurements performed by the UE are then reported to the network such as the network node, which may use them for various tasks.
The radio network node e.g. a base station may also perform signal measurements. Examples of radio network node measurements in LTE are propagation delay between UE and itself, UL Signal to Interference plus Noise Ratio (SINR), UL Signal-to-Noise Ratio SNR, UL signal strength, Received Interference Power (RIP) etc. The radio network node such as an eNB may also perform positioning measurements which are described in a later section.
Radio Link Monitoring Measurements
The UE also performs measurements on the serving cell also known as primary cell or PCell, in order to monitor the serving cell performance. This is called as Radio Link Monitoring (RLM) or RLM related measurements in LTE.
For RLM the UE monitors the downlink link quality based on the cell-specific reference signal in order to detect the downlink radio link quality of the serving or PCell.
In order to detect out of synchronization and in synchronization, the UE compares the estimated quality (Q) with thresholds Qout and Qin respectively. The threshold Qout and Qin are defined as the level at which the downlink radio link cannot be reliably received and corresponds to 10% and 2% block error rate of a hypothetical PDCCH transmissions respectively.
In non-Discontinuous Reception (DRX), downlink link quality for out of sync and in sync are estimated over an evaluation periods of 200 ms and 100 ms respectively.
In DRX, downlink link quality for out of sync and in sync are estimated over the same evaluation period, which scale with the DRX cycle e.g. period equal to 20 DRX cycles for DRX cycle greater than 10 ms and up to 40 ms.
In non-DRX, the out of sync and in sync status are assessed by the UE in every radio frame. In DRX the out of sync and in sync status are assessed by the UE once every DRX.
In addition to filtering on physical layer i.e. evaluation period, the UE also applies higher layer filtering based on network configured parameters. This increases the reliability of radio link failure detection and thus avoids unnecessary radio link failure and consequently RRC re-establishment. The higher layer filtering for radio link failure and recovery detection would in general comprise the following network controlled parameters:                Hysteresis counters e.g. N310 and N311 out of sync and in sync counters respectively.        Timers e.g. T310 RLF timer        
For example the UE starts the timer T310 after N310 consecutive Out Of Synchronization (OOS) detections. The UE stops the timer T310 after N311 consecutive IS detections. The transmitter power of the UE is turned off within 40 ms after the expiry of T310 timer. Upon expiry of T310 timer the UE starts T311 timer. Upon T311 expiry the UE initiates RRC re-establishment phase during which it reselects a new strongest cell.
In HSPA similar concept called out of sync and in sync detection are carried out by the UE. The higher layer filtering parameters (i.e. hysteresis counters and timers) are also used in HSPA. There is also Radio Link Failure (RLF) and eventually RRC re-establishment procedures specified in HSPA.
Sampling of Cell Measurement
The overall serving cell or neighbour cell measurement quantity results comprises of non-coherent averaging of 2 or more basic non-coherent averaged samples. The exact sampling depends upon the implementation and is generally not specified. An example of RSRP measurement averaging in E-UTRAN is shown in FIG. 3. FIG. 3 illustrates an Example of RSRP measurement averaging in E-UTRAN, wherein the UE obtains the overall measurement quantity result by collecting four non-coherent averaged samples or snapshots, each of 3 ms length in this example, during the physical layer measurement period, i.e. 200 ms, when no DRX is used or when DRX cycle is not larger than 40 ms. Every coherent averaged sample is 1 ms long. The measurement accuracy of the neighbour cell measurement quantity, e.g. RSRP or RSRQ, is specified over this physical layer measurement period. It should be noted that the sampling rate is UE implementation specific. Therefore in another implementation a UE may use only 3 snap shots over 200 ms interval. Regardless of the sampling rate, it is important that the measured quantity fulfils the performance requirements in terms of the specified measurement accuracy.
In case of RSRQ both RSRP, numerator, and carrier RSSI, denominator, should be sampled at the same time to follow similar fading profile on both components. The sampling also depends upon the length of the DRX cycle. For example for DRX cycle>40 ms, the UE typically takes one sample every DRX cycle over the measurement period.
A similar measurement sampling mechanism is used for other signal measurements by the UE and also by the BS for UL measurements.
Positioning
Several positioning methods for determining the location of the target device, which can be a UE, mobile relay, PDA etc. exist. The methods are:                Satellite based methods; it uses A-GNSS (e.g. A-GPS) measurements for determining UE position        Observed Time Difference Of Arrival (OTDOA); it uses UE Reference Signal Time Difference (RSTD) measurement for determining UE position in LTE        Uplink-Time Difference of Arrival (UTDOA); it uses measurements done at LMU for determining UE position        Enhanced cell ID; it uses one or more of UE Rx-Tx time difference, BS Rx-Tx time difference, LTE RSRP/RSRQ, HSPA CPICH measurements, Angle of Arrival (AoA) etc for determining UE position. Fingerprinting is considered to be one type of enhanced cell ID method.        Hybrid methods; it uses measurements from more than one method for determining UE position.        
In LTE the positioning node also known as E-SMLC or location server, configures the UE, eNodeB or Location Measurement Unit (LMU) to perform one or more positioning measurements. The positioning measurements are used by the UE or positioning node to determine the UE location. The positioning node communicates with UE and eNode B in LTE using LTE Positioning Protocol (LPP) and LPPa protocols respectively.
As mentioned above, In LTE NCT, arrangements of reference signals are different than the 3GPP Release 8 LTE carriers, LCT. In case of LCT, CRS also known as RS are transmitted by a network node in the DL in every DL subframe and over entire DL cell BW. However in case of cells on NCT the CRS are not transmitted in every DL subframe and may also be sent over limited cell BW.
The NCT carrier may also contain mixture of cells i.e. some operate using LCT and some operate using NCT. This will degrade the measurements performed by a UE on cells on such a “mixed NCT carrier”.
Please note that the wordings “in cells” and “on cells” have an equal meaning and are used interchangeably in this document.